Re: [Qemu-devel] [Nbd] [PATCH 1/2] NBD proto: add WRITE_ZEROES extension

[Paolo Bonzini]

On 24/03/2016 09:26, Wouter Verhelst wrote:
>> > 
>> > No, there is no specific reason. Looks like NBD_CMD_FLAG_ZEROES fits the
>> > spec and implementations nicely. So I'll rewrite the extension and add
>> > the flag instead of the whole command.
> Actually, having given this some more thought...
> 
> There is at least one server-side implementation of nbd (mine) which
> silently ignores flags it doesn't know about. This isn't a problem for
> non-critical flags, but it could be a problem for a flag like this. Of
> course, a client shouldn't send a flag to a server which that server
> hasn't heard of, but mistakes do happen.
> 
> Do we want to keep that in mind? If so, we might want to keep it as a
> separate command after all.
> 
> OTOH, it could be said that silently ignoring unknown messages is a bug.
> I should probably just fix my implementation instead.

Even if it is a bug, it does suggest that the payload format should not
be changed by flags.  For example ignoring flags is a bug for an NBD
server, but not for a Wireshark protocol dissector.

Paolo